1. Field
The invention relates to a method and system for controlling a radio communications network and a radio network controller. Particularly the invention relates to the handover procedure in a cellular system. The invention can be advantageously applied in broadband radio networks that offer fixed network services to their users.
2. Brief Description of Related Developments
Below it will be described the prior art by first illustrating the operation of a popular second-generation cellular system and in particular the handover, or change of active base stations serving a mobile station moving in the cellular network's coverage area. Then it will be disclosed the characteristics of new, third-generation cellular systems and problems related to prior-art handover solutions.
Prior Art; Second-generation Cellular Systems
A terminal of a cellular radio system attempts to choose a base station so as to operate on said base station's coverage area, or cell. Conventionally, the choice has been based on the measurement of the strength of the received radio signal in the terminal and base station. For example, in GSM (Global System for Mobile telecommunications) each base station transmits a signal on a so-called broadcast control channel (BCCH) and the terminals measure the strengths of the received BCCH signals and based on that, determine which cell is the most advantageous one as regards the quality of the radio link. Base stations also transmit to the terminals information about the BCCH frequencies used in the neighbouring cells so that the terminals know what frequencies they have to listen to in order to find the BCCH transmissions of the neighbouring cells.
FIG. 1 shows a second-generation cellular system that comprises a mobile switching centre (MSC) belonging to the core network (CN) of the cellular system as well as base station controllers (BSC) and base stations (BS) belonging to the a radio access network (RAN), to which mobile stations (MS) are linked via radio interface. FIG. 2 shows the coverage areas C21-C29 of base stations BS21-BS29 of a second-generation cellular system.
In second-generation cellular systems, such as GSM, communication between base stations BS and the core network CN occurs via base station controllers BSC. Usually, one base station controller controls a large number of base stations so that when a terminal moves from the area of a cell to the area of another cell, the base stations of both the old and the new cell are connected to the same base station controller. Thus the handover can be executed in the base station controller. So, in the conventional GSM system, for example, there occur fairly few handovers between a base station of a first base station controller and a base station of a second base station controller. In such a case, the switching centre has to release the connection with the first base station controller and establish a new connection with the new base station controller.
Such an event involves a lot of signaling between the base station controllers and the switching centre and as the distances between the base station controllers and the switching centre may be long there may occur disturbances in the connection during the handover.
Third-generation Cellular Systems
The prior-art handover arrangement is suitable for the so-called second-generation digital cellular radio systems such as GSM and its extension DCS 1800 (Digital Communications System at 1800 MHz), IS-54 (Interim Standard 54), and PDC (Personal Digital Cellular). However, it has been suggested that in future third-generation digital cellular systems the service levels offered to the terminals by the cells may differ considerably from a cell to another. Proposals for third-generation systems include UMTS (Universal Mobile Telecommunications System) and FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz). In these plans cells are categorised according to their size and characteristics into pico-, nano-, micro- and macrocells, and an example of the service level is the bit rate. The bit rate is the highest in picocells and the lowest in macrocells. The cells may overlap partially or completely and there may be different terminals so that not all terminals necessarily are able to utilise all the service levels offered by the cells.
FIG. 3 shows a version of a future cellular radio system which is not entirely new compared with the known GSM system but which includes both known elements and completely new elements. In current cellular radio systems the bottleneck that prevents more advanced services from being offered to the terminals comprises the radio access network RAN which includes the base stations and base station controllers. The core network of a cellular radio system comprises mobile services switching centres (MSC), other network elements (in GSM, e.g. SGSN and GGSN, i.e. Serving GPRS Support Node and Gateway GPRS Support node, where GPRS stands for General Packet Radio Service) and the related transmission systems. According e.g. to the GSM+ specifications developed from GSM the core network can also provide new services.
In FIG. 3, the core network of a cellular radio system 30 comprises a GSM+ core network 31 which has three parallel radio access networks linked to it. Of those, networks 32 and 33 are UMTS radio access networks and network 34 is a GSM+ radio access network. The upper UMTS radio access network 32 is e.g. a commercial radio access network, owned by a telecommunications operator offering mobile services, which equally serves all subscribers of said telecommunications operator. The lower UMTS radio access network 33 is e.g. private and owned e.g. by a company in whose premises said radio access network operates. Typically the cells of the private radio access network 33 are nano- and/or picocells in which only terminals of the employees of said company can operate. All three radio access networks may have cells of different sizes offering different types of services. Additionally, cells of all three radio access networks 32, 33 and 34 may overlap either entirely or in part. The bit rate used at a given moment of time depends, among other things, on the radio path conditions, characteristics of the services used, regional overall capacity of the cellular system and the capacity needs of other users. The new types of radio access networks mentioned above are called generic radio access networks (GRAN). Such a network can co-operate with different types of fixed core networks CN and especially with the GPRS network of the GSM system. The generic radio access network (GRAN) can be defined as a set of base stations (BS) and radio network controllers (RNC) that are capable of communicating with each other using signaling messages. Below, the generic radio access network will be called in short a radio network GRAN.
The terminal 35 shown in FIG. 3 is preferably a so-called dual-mode terminal that can serve either as a second-generation GSM terminal or as a third-generation UMTS terminal according to what kind of services are available at each particular location and what the user's communication needs are. It may also be a multimode terminal that can function as terminal of several different communications systems according to need and the services available. Radio access networks and services available to the user are specified in a subscriber identity module 36 (SIM) connected to the terminal.
FIG. 4 shows in more detail a core network CN of a third-generation cellular system, comprising a switching centre MSC, and a radio network GRAN connected to the core network. The radio network GRAN comprises radio network controllers RNC and base stations BS connected to them. A given radio network controller RNC and the base stations connected to it are able to offer broadband services while a second radio network controller and base stations connected to it may be able to offer only conventional narrowband services but possibly covering a larger area.
FIG. 5 shows coverage areas 51a-56a of base stations 51-56 in a third-generation cellular system. As can be seen from FIG. 5, a mobile station travelling only a short distance can choose from many base stations for the radio link.
New cellular systems can employ a so-called macrodiversity combining technique related to CDMA systems. This means that on the downlink path a terminal receives user data from at least two base stations and correspondingly, the user data transmitted by the terminal is received by at least two base stations. Then, instead of one, there are two or more active base stations, or a so-called active set. Using macrodiversity combining it is possible to achieve a better quality of data communications as momentary fade-outs and disturbances occurring on a given transmission path can be compensated for by means of data transmitted via a second transmission path.
For selecting an active set an active radio network controller determines, on the basis of the geographic location, for example, a candidate set of base stations, which is a set of the base stations that are used for measuring general signal strength information using e.g. a pilot signal. Below, this candidate set of base stations will be called a candidate set (CS) in short. In some systems, such as IS-41, separate candidate base stations are used.
Problems Related to Prior Art
Let us consider the application of a prior-art arrangement to a proposed third-generation digital cellular system. In third-generation systems, base station handovers and radio network controller handovers are more frequent than in second-generation systems. One of the reasons behind this is that the cell sizes may be remarkably small and that there may occur need to change the service type e.g. from narrowband to broadband during a call.
In accordance with the prior art a handover between radio network controllers would be carried out in such a manner that the user data connection between the switching centre and the so-called old active radio network controller/base station is released and a new connection is established between the switching centre and the so-called new active radio network controller/base station. Then the switching centre would have to release/set up many connections, which involves a lot of signaling between the switching centre and the radio network controller. Furthermore, there are very many small-sized cells in the area of one switching centre, and in broadband applications the amount of user data transmitted is great. This puts very tight requirements for capacity and speed on the switching centre hardware, which in large systems cannot be met at reasonable costs using current technology.
Secondly, known systems have a problem of how to transmit signaling and data of the core network CN and signaling of the radio network to a terminal moving in the radio network's area. CN signaling and data are specifically meant for the terminal and routed via radio network controllers. Radio network signaling may be intended either for the terminal or for the radio network itself so that it can arrange optimal use of radio resources in the network area. The problem is caused by the moving terminal and its effect on the flow of data in the radio network's area.
When using macrodiversity combining the prior art further has the problem that after a handover between radio network controllers the new radio network controller does not have knowledge of the base stations suitable for macrodiversity combining so that macrodiversity combining cannot be used before the new radio network controller has established a candidate set of its own. Therefore, transmission power has to be increased and only one transmission path can be used temporarily between the system and the terminal. This degrades the quality of communications and causes stability problems which must be corrected by constant adjustments.